Generally, catalytically active materials are applied to highly porous materials in more or less fine distributions. On the one hand this is done because many catalytically active materials sinter quickly at the temperatures at which the catalyst must be pre-treated or used. On the other hand one must use carrier materials in the case of very expensive catalytically active materials, such as precious metals, for example platinum, palladium, or ruthenium. In those cases the aim is to obtain a maximum number of atoms of the catalytically active component on the surface of the catalytically active particles. Therefore, these catalytically active materials are often used as particles with sizes of about 1 nm. When expensive catalytically active materials, such as precious metals, are used, carbon based carriers are very attractive. Once the catalyst is deactivated, the precious metal can easily be reclaimed by oxidizing the carbon carrier. After oxidation the precious metal remains, and can be used again.
The mechanical strength of the catalyst carriers is of great importance in their application. This applies first of all to the use of catalysts suspended in a liquid phase. To keep the catalytically active particles well dispersed in the liquid, the liquid must be agitated vigorously. Furthermore, the catalyst must be separated from the liquid, for example by filtration or centrifugation, at the completion of the reaction. During this, carrier particles with a low mechanical strength will disintegrate, yielding extremely small particles. At the current state-of-the-art such particles cannot easily be separated by filtration or centrifugation. In particular when precious metals are used as the catalytically active materials this is unacceptable, because it will result in unacceptable losses of the expensive precious metal. The mechanical strength is also of great importance when the catalyst is used as a fixed bed catalyst. During the introduction of the catalyst bodies in the reactor virtually no attrition or dust generation is allowed. Dust generation results in a large pressure drop over the catalyst bed, while small attrition-generated particles are entrained by the reactant flow that is passed through the reactor. The formation of small particles by attrition also yields a catalyst that shows a non-stable performance in time. Very often the selectivity decreases. Both effects are unwanted, since a stable performance is necessary from a controlling and safety aspect.
According to the general state-of-the-art, activated carbon is used as the carbon carrier. Activated carbon is manufactured from natural materials, such as wood or peat. This is objectionable, because generally the characteristics of the activated carbon obtained from such materials are hard to control. Providing bodies comprising active carbon with constant, well-adjustable characteristics is therefore a known problem that hasn't been satisfactorily solved until now. An additional objection is the fact that, in the presence of (small amounts of) surfactants, such as detergents, in the liquid the catalyst particles must be dispersed in, catalyst bodies produced from activated carbon might disintegrate quickly.
In the case of bodies intended for a fixed catalyst bed, in which no attrition is allowed, the most obvious possibility is the use of carbon obtained by a thermal treatment of coconut shells. This provides very tough and mechanically strong bodies. However, the fact that the accessible surface of carbon obtained by thermal decomposition of coconut shells is small, is a drawback. As a result the obtainable catalytically active surface per unit volume is relatively small.
The final drawback of carrier materials produced from natural starting materials is their chemical composition. Natural materials often contain elements such as potassium, magnesium, calcium, and sulphur, which could cause problems during the use as catalyst or the recycling of the precious metals. There is, therefore, a strong technological need for carbon based carriers with a high mechanical strength and an extremely well controlled chemical composition that can consequently be produced from a source wherein the properties of which can be better controlled than from peat and wood starting materials.
It has been proposed to manufacture such catalyst carriers from carbon nanofibers or nanotubes. In WO 93/24214 (Hyperion) it is proposed to use carbon nanofibers or nanotubes as catalyst carriers in which the graphitic layers are oriented essentially in parallel to the filament axis. The use of such relatively long and straight carbon filaments as bodies with controllable dimensions is difficult.
The bodies of catalysts to be employed in a fixed catalyst bed must have a minimum size of about 1 mm. The pressure drop with smaller particles is too high with technical applications. It has proven to be very difficult to manufacture mechanically strong bodies from these sizes from the carbon nanofibers or nanotubes described above.
Indeed, for catalysts the dimensions and porosity are of great importance. In fixed catalyst beds the dimensions of the carrier bodies determine the pressure drop and the transport of reactants and reaction products through the catalyst bodies. In the case of liquid-suspended catalysts the transport of the reactants and reaction products is of great importance. The dimensions of the catalyst bodies are, as indicated above, of great importance to these transports, as well as to the separation of the bodies, for example by filtration of centrifugation. Another drawback is the fact that carbon nanofibers or nanotubes must be grown from metallic particles applied on carriers such as silicon dioxide or aluminum oxide. These carriers can often interfere with the application of the obtained carbon carriers in liquid phase reactions.
It has been proposed to manufacture carbon-based carrier bodies by thermal decomposition of spheres of microcrystalline cellulose WO 2007/131795 (Glatt). Such spheres are known in the state-of-the-art for the controlled release of medicinal compounds (“slow release”). It was found that carbon spheres with a very high mechanical strength could be produced this way. Considering the fact that microcrystalline cellulose spheres with dimensions of about 0.1 to 0.7 mm are produced industrially, the above carbon spheres can be manufactured with a consistent quality.
A drawback of microcrystalline cellulose is its high price. During the thermal treatment of the microcrystalline cellulose spheres their weight decreases by 80%. This means that the cost per unit weight of the carrier in comparison to carbon spheres obtained according to the state-of-the-art becomes very high.